VERTEBRATES. 73 



and highly polished, without striae or pores; superior margin 

 forming a sharp, cutting edge, divided into numerous lobes, of 

 which the middle one is largest, the others diminishing succes- 

 sively, to the lateral angles. These lobes are usually unequal 

 in number on either side of the largest one, four on one side 

 and five on the other. They are generally rounded in outline 

 but sometimes acute. The anterior and posterior surfaces of 

 the crown are of nearly equal height, the anterior sometimes 

 showing a basal ridge as in Petalodus. The root is thicker than 

 the crown and higher, nearly or quite as broad and of a some- 

 what similar form, except that its border is not scolloped. 

 Both faces of the root are more or less roughened. On the 

 posterior surface the root at its junction with the crown forms 

 a broad shoulder, which is depressed at the centre with a deep, 

 conical cavity. This gives to the profile section much the 

 character of that of some species Petalodus, as will be seen by 

 a reference to the figures. 



When viewed from the front or rear the outline of these seems strikingly 

 like that of the teeth of Ct. apicalis, Ag., as given by Agassiz, Poiss. Foss. 

 Atlas, Vol. 3, tab. I'd, fig. 1, la. Yet it would seem that in fact these 

 forms are so widely different that it is even doubtful if they should be included 

 in the same genus. Ct. apicalis is described as "transversely ovate," and 

 judging from the figures and description of Agassiz and McCoy must be some- 

 what similar in general aspect to Chomatodus dentatus, McCoy, while the teeth 

 before us are thinner and more curved laterally than in any species of Petalodus. 

 Ct. serratus, Owen, {Brit. Palseoz. Poss. p. 626, PI. 3-1, fig. 21, 22, 23), is, how- 

 ever, more like our species in general form, but is less curved and has nearly 

 thirty denticles on the cutting edge, while in Ct. semicircularis there are but 

 seven or eight. 



The affinity of this species of Ctenoptycliius with Petalodus will be seen by 

 reference to the figures, to be very marked. McCoy has noticea this relation- 

 ship in his description of Ct. serratus, which would be considered a Petalodus 

 were it not for the coarse crenulations of the cutting edge. M. Agassiz com- 

 pares Ct. apicalis to a small, compressed Orodus, which it is certainly very like, 

 but it is evident that the present species is more closely allied to Petalodus than 

 Orodus, and is in fact the most exaggerated form of lamillar and arched type 

 of teeth first and best known by the former name. 



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